An onsen(温泉?) is a term for hot springs in the Japanese language, though the term is often used to describe the bathing facilities and inns around the hot springs. As a volcanically active country, Japan has thousands of onsen scattered along its length and breadth. Onsen were traditionally used as public bathing places and today play a central role in directing Japanese domestic tourism.

Onsen come in many types and shapes, including outdoor (露天風呂 or 野天風呂,roten-buro or noten-buro?) and indoor baths. Baths may be either public run by a municipality or private (内湯,uchiyu?) often run as part of a hotel, ryokan or bed and breakfast(民宿,minshuku?).

Onsen are a central feature of Japanese tourism often found out in the countryside but there are a number of popular establishments still found within major cities. They are a major tourist attraction drawing Japanese couples, families or company groups who want to get away from the hectic life of the city to relax. Japanese often talk of the virtues of "naked communion" (裸の付き合い,hadaka no tsukiai?)[1] for breaking down barriers and getting to know people in the relaxed homey atmosphere of a ryokan with an attached onsen. Japanese television channels often feature special programs about local onsens.[citation needed]

The presence of an onsen is often indicated on signs and maps by the symbol ♨ or the kanji, 湯 (yu, meaning "hot water"). Sometimes the simpler hiragana character ゆ (yu) is used, to be understandable to younger children.[citation needed]

Traditionally, onsen were located outdoors, although a large number of inns have now built indoor bathing facilities as well. Onsen by definition use naturally hot water from geothermally heated springs. Onsen should be differentiated from sentō, indoor public bath houses where the baths are filled with heated tap water.[citation needed]

The legal definition of an onsen includes that its water must contain at least one of 19 designated chemical elements, including such minerals as iron, sulfur, and metabolic acid and be 25 °C or warmer before being reheated. Stratifications exist for waters of different temperatures. Major onsen resort hotels often feature a wide variety of themed spa baths and artificial waterfalls in the bathing area utaseyu(打たせ湯?).

Onsen water is believed to have healing powers derived from its mineral content. A particular onsen may feature several different baths, each with water with a different mineral composition. The outdoor bath tubs are most often made from Japanese cypress, marble or granite, while indoor tubs may be made with tile, acrylic glass or stainless steel. Different onsen also boast about their different waters or mineral compositions, plus what healing properties these may contain. Other services like massages may be offered.[citation needed]

People often travel to onsen with work colleagues, friends, couples or their families.[citation needed]

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Traditionally, men and women bathed together at the onsen and sentō but gender separation has been enforced since the opening of Japan to the West during the Meiji Restoration. Mixed bathing(混浴,konyoku?) persists at some special onsen in rural areas of Japan,[2] which usually also provide the option of separate "women-only" baths or different hours for the two sexes. Men usually cover their genitals with a small towel while bathing, while women usually wrap their bodies in full size towels. Children of either sex may be seen in both the men's and the women's baths. In some prefectures of Japan, including Tokyo, where nude mixed bathing is banned, people are obligated to wear swimsuits or yugi (湯着,yugi?), which are specialized clothing for baths.

At an onsen, as at a sentō, all guests are expected to wash their bodies and rinse themselves thoroughly before entering the hot water. Bathing stations are equipped with stools, faucets, wooden buckets, and toiletries such as soap and shampoo; nearly all onsen also provide removable shower heads for bathing convenience. Entering the onsen while still dirty or with traces of soap on the body is socially unacceptable.[3]

Bathers are not normally allowed to wear swimsuits in the baths. However, some modern onsen with more of a waterpark atmosphere require their guests to wear a swimming suit in their mixed baths.[citation needed]

Onsen guests generally bring a small towel with them to use as a wash cloth. The towel can also provide a modicum of modesty when walking between the washing area and the baths. Some onsen allow one to wear the towel into the baths, while others have posted signs prohibiting this, saying that it makes it harder to clean the bath. It is sometimes against the rules to immerse or dip towels in the onsen bath water, since this can be considered unclean. In this latter case, people normally set their towels off to the side of the water when enjoying the baths, or place their folded towels on top of their heads.[citation needed]

Onsen vary from quiet to noisy, some play piped music and often feature gushing fountains. Bathers will engage in conversation in this relaxed situation. There are usually prohibitions against rowdiness in the washing and bathing areas. A small amount of excess energy and splashing around is usually tolerated from children, however.[citation needed]

Many onsen ban bathers with tattoos, which in Japan, as in the West prior to the radical changes that have taken place in society, are recognized as a badge of criminality—Yakuza traditionally have elaborate tattoos. Despite this background reason, the rule is often enforced strictly against all, including foreigners, women, and even when tattoos are small and "peaceful".[4][5][6]

In Japan, it is said onsen have various medical effects.[7] Japanese people believe that a good soak in proper onsen heals aches, pains and diseases, and visit onsen to treat the illnesses, such as arthralgia, chronic skin diseases, diabetes, constipation, menstrual disorders and so on.[7]

These medical benefits have given onsen a central role in balneotherapy which is called "Onsen Therapy" (温泉療法,onsen-ryōhō?). Onsen Therapy is a comprehensive bathing treatment conducted to maintain health, normalize dysfunctions and prevent illness.[7]

Although millions of Japanese bathe in onsen every year with few noticeable side effects, there are still contraindications to onsen usage, such as high blood pressure or heart disease.[8]

Legionella bacteria have been found sporadically in onsen with poor sanitation.[9][10] Revelations of poor sanitary practices at some onsen have led to improved regulation by hot spring communities to maintain their reputation.[11]

There have been reports of infectious disease found in hot bodies of water worldwide, such as:

Various Naegleria species.[12] While studies have found the presence of Naegleria in hot spring waters, the worrisome Naegleria fowleri amoeba has not been identified.[12] Nevertheless, less than 5 cases have been seen historically in Japan, although not conclusively linked to onsen exposure.[13]

Many onsen have posted notices for visitors, reminding anyone with open cuts, sores, or lesions to not bathe. Additionally, in recent years onsen are increasingly adding chlorine to their waters to prevent infection, although many onsen purists seek natural, unchlorinated onsen that instead does not recycle its water, cleaning baths daily.[11] These precautions as well as proper onsen usage (i.e. not placing the head underwater, washing thoroughly before entering the bath) greatly reduces any overall risk to bathers.